Paying the Broker

The broker is paid by whoever has promised to pay.  In most cases, that's the seller of the property.  Some buyers hire their own agents, usually with a promise to pay that broker if it turns out the seller has not offered a share of commission to whichever agent brings the buyer.

You mention "paying a realator".  Only a licensed broker can collect commissions.  An agent who is licensed as a salesperson is working more or less as an apprentice under the supervision of a broker who will collect whatever fees are due on a transaction.  A Realtor is a broker who chooses to join a private trade organization known as the National Association of Realtors.